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Traditionally, power system's inertia has been estimated according to the rotating masses directly connected to the grid. However, a new generation mix scenario is currently identified, where conventional supply-side is gradually replaced by renewable sources decoupled from the grid by electronic converters (i.e., wind and photovoltaic power plants). Due to the significant penetration of such renewable generation units, the conventional grid inertia is decreasing, subsequently affecting both reliability analysis and grid stability. As a result, concepts such as 'synthetic inertia', 'hidden inertia' or 'virtual inertia', together with alternative spinning reserves, are currently under discussion to ensure power system stability and reliability. Under this new framework, an algorithm to estimate the minimum inertia needed to fulfil the ENTSO-E requirements for ROCOF values is proposed and assessed under a relevant variety of imbalanced conditions. The additional active power needed to be within the frequency dynamic range is also estimated and determined. Both inertia and additional active power can come from different sources, such as storage solutions, renewable sources decoupled from the grid including some frequency control strategies, interconnections with other grids, or a combination of them. The power system under consideration includes thermal, hydro-power plants, and renewable generation units, in line with most current and future European supply-side power systems. More than 700 generation mix scenarios are identified and simulated, varying the renewable integration, the power imbalance, and the inertia constant of conventional power plants. In fact, the solutions studied here provide important information to ease the massive integration of renewable resources, without reducing the capacity of the grid in terms of stability and response to contingencies.